1. Field of the Invention
The invention relates to a method and a device for controlling the level of a pulsed high-frequency signal.
2. Discussion of the Background
Pulsed high-frequency signals can be generated in signal generators. In this context, a high-frequency signal generated by a frequency synthesiser is supplied to a pulse modulator which is controlled by a pulse signal. For the duration of a single pulse of the pulse signal, the high-frequency signal is conducted from the input to the output of the pulse modulator. By contrast, for the duration of the single pulse pauses of the pulse signal, the high-frequency signal is not conducted from the input to the output of the pulse modulator.
In controlling a level of a pulsed high-frequency signal, the signal level of the pulsed high-frequency signal is registered at the output of the pulse modulator with a detector and compared with a reference value. Dependent upon this control difference, an adjustment signal is generated with a level controller, with which the amplification or respectively attenuation factor of an amplification or attenuation element disposed upstream of the pulse modulator is adjusted. In this manner, the signal level of the pulsed high-frequency signal is controlled to the desired reference value. Because of the time dynamic of the detector, the level controller and the amplification or attenuation element, such a level control provides a limited control bandwidth, which typically extends up to a few megahertz.
During the pulse pauses of the pulse signal, the signal level of the pulsed high-frequency signal provides a value of zero, or approximately zero, and can therefore not be registered by the detector. Accordingly, the level control cannot meaningfully operate during the pulse pauses of the pulse signal. Opening the pulse control during the pulse pauses of the pulse signal is known from U.S. Pat. No. 5,054,116.
Conversely, because of its limited control bandwidth, it is not possible for the level control to complete the control process fully to its conclusion in short pulse pauses of the pulse signal. Accordingly, in the case of pulsed high-frequency signals, a level control in pulse mode often takes place only in comparatively long pulses of the pulse signal or not at all. For example, if the signal level of the pulsed high-frequency signal changes, for example, because of temperature drift or another influence, the pulse mode is cancelled, and the level control for the un-pulsed high-frequency signal is activated. If the level control of the un-pulsed high-frequency signal reaches the specified reference value, the level control is switched off, the value of the present adjustment signal is held constant in the level controller, and the pulse mode is reactivated. However, level changes of the pulsed high-frequency signal disadvantageously remain in existence until the next level control is activated. Moreover, supplying a device under test with an un-pulsed high-frequency signal during a level control of relatively long duration can cause undesirable damage to the device under test.